Monday, November 2, 2015

When we talked about clouds and precipitation. We mentioned that when air is
forced to rise, it expands and cools and water vapor is squeezed out of the air
by the process of condensation. The moisture could take the form of dew, frost,
fog, or precipitation.We mentioned
various places where air is forced to rise such as mountains, along fronts and
in storms.Another place air is forced
to rise is in the doldrums.This is an
area of low pressure located near the equator of the earth. The northeast trade
winds in the northern hemisphere converge with the southeast trades of the
southern hemisphere.Where these winds
come together, the air is forced to rise. As a result, this is an area of heavy
rainfall day after day.This
Doldrum-belt does shift during the year.In the summertime it migrates about five degrees north of the
equator.During the autumn and spring
season it is located very close to the equator and in the wintertime it
migrates south of the equator.

Just the opposite, the horse latitude belt is where the
northeast trade winds originate. Air flows away from the horse latitude
belt.At the same time, the
south-westerlies originate from the horse latitude belt and flow away from the
area.As air flows away from the horse latitude belt, air must descend
to take the place of the departing surface air.Air is sinking, thus warming and no clouds or precipitation occur.The horse latitude belt is located about 30
degrees north and south of the equator.For the most part, this part of the earth is void of clouds, wind and
precipitation.In the early days,
explorers had to lighten the loads on their ships because there was no wind to
push them along.Everything had to go
overboard, even the horses they brought along for transportation in the new
world.As a result, this area became
known as the horse latitudes.

Up
at 60 degrees north there is another low- pressure area like the doldrums.Here we see the convergence of the prevailing
southwesterlies and the polar northeasterlies.Again, as they converge air is forced to rise and an area of clouds and
precipitation are formed.This area is
called the subpolar low area.What is
important is this, where the air is forced to rise, clouds and precipitation
prevails and where air is sinking, there is little in the way of clouds or
precipitation.

Now
that we know where and why

Precipitation
occurs, let’s look at the different forms of precipitation.First, there is rain. This is described as
drops of water falling from clouds at least a few thousand feet above the
ground.Very fine drops, called drizzle,
falls from lower clouds such as stratus clouds. The classic snowflake consists
of a six-sided crystal.At very low
temperatures snow may take the form of ice needles.Sleet or ice pellets are formed when
raindrops fall through below-freezing layers of air, solidifying into pellets
of clear ice. Freezing rain forms when drops of rain come in contact with
below-freezing surfaces on the ground.This
condition, if prolonged, may create an ice storm, causing hazardous driving
conditions along with severe damage to trees, shrubs, and telephone and
electric wires.Finally, hail can be
described as alternating layers of snow and ice that resemble an onion in
structure.Hailstones, oddly enough, are
usually associated with severe summer thunderstorms and are rarely experienced
during the winter.

The
type of precipitation that strikes the ground depends on the vertical and
thermal structure of

the
atmosphere. If the clouds are located completely in the cold air (below 32
degrees) then the temperature of the dew point will be below freezing and snow
forms. As snow falls through the air and hits the ground, it does not melt
since the ground temperature is also below freezing.The snow is said to stick.

If
the clouds are located both in the cold air and the warm air, then raindrops
are formed in the warm air while snow is formed in the cold air.As the raindrops fall into the colder air
below, they eventually freeze and form solid pellets of ice which we call sleet
or ice pellets. These pellets reach the ground as sleet, which some people
mistakenly call hail. So, where the cloud is located in both warm air and cold
air, we get a mixture of snow and sleet. If the entire cloud exists in above
freezing warm air, no snowflakes can form, only raindrops.If, however, the cold air below is thick
enough, rain drops could form pellets of ice and sleet would form, however, if
the cold air below the clouds is extremely shallow, then the rain drops
will remain rain drops until they actually hit the ground.If the ground temperature is below 32
degrees, then the rain freezes on contact with the ground, forming freezing
rain or glaze.

What
forms of precipitation hits the ground if the entire cloud is located in the
warm air (above 32 degrees) and the surface temperature is also above 32
degrees?